رَجُلٌ عَالِمٌ: the second indefinite word can be a direct adjective.
الرَّجُلُ عَالِمٌ: the indefinite ʿālimun is a predicate of a definite noun.
مَرَرْتُ بِرَجُلٍ أَبُوهُ عَالِمٌ: the entire nominal clause is a sentence adjective in genitive position.
Same lexical meaning does not entail the same syntactic analysis.
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The Tawābiʿ and Naʿt: Direct and Relational Attributes
رَجُلٌ عَالِمٌ: the second indefinite word can be a direct adjective.
الرَّجُلُ عَالِمٌ: the indefinite ʿālimun is a predicate of a definite noun.
مَرَرْتُ بِرَجُلٍ أَبُوهُ عَالِمٌ: the entire nominal clause is a sentence adjective in genitive position.
Same lexical meaning does not entail the same syntactic analysis.